(1) Field of the Invention
The present invention relates generally to an apparatus and method for discharging ash from fossil-fired boiler hoppers, and in particular to an apparatus and method for controlling the discharge sequence of a plurality of ash-collection hoppers to optimize discharge and minimize unnecessary hopper activation.
(2) Description of the Prior Art
A large quantity of bottom ash and fly ash is produced when burning coal in steam boilers. Bottom ash is generally the larger, heavier ash that falls downward from the furnace area, while fly ash is the fine, fluffy air-borne ash that is removed from exhaust gases, usually with electrostatic precipitators, and later collected for disposal. Generally, both types of ash fall, or are conveyed, to a plurality of adjacent hoppers that are positioned in the lower part of the boiler to collect bottom ash, and below the precipitator to collect fly ash. Depending upon the boiler design, as many as forty hoppers may be used for ash collection.
These hoppers are generally comprised of an ash container with inwardly angled sidewalls having lower edges that terminate in a discharge opening, and a moveable gate valve that closes the opening. The hopper is periodically discharged by opening the valve to transfer the ash to an ash conveyor, such as belt or conduit. For example, the gate valve may connect the hopper to a conduit that includes a vacuum source to draw the ash from the hopper and through the conduit to convey the ash to a remote location for further processing or disposal.
Opening of the gate valve is controlled by an actuator, e.g., an electrical or pneumatic actuator, to move the gate valve between closed and open positions. Each hopper valve has a separate actuator, so that the valves of multiple hoppers are individually controlled. In normal operation, only one of a plurality of hoppers is discharged at a given time so that downstream components, e.g., conduits and vacuum devices, can be economically sized to one hopper capacity.
Sequential opening of the hopper valves is effected by controlling the valve actuators with a discharge controller that is set to open each of the hoppers individually in sequence. That is, the valve of a first hopper is opened and the hopper is discharged. After the first hopper valve is returned to the closed position, the valve of the next hopper in sequence is opened. This sequential opening is continued until all of the hoppers have been discharged, and the opening cycle is then repeated.
In many boilers, each hopper is maintained in the open position for a predetermined time, even if the hopper is discharged in a lesser time. Thus, the hopper may remain open for a significant time with no ash being discharged from the hopper, reducing the overall efficiency of the system, and accelerating deterioration by exposing the hopper to hot gases.
In other boilers, one or more vacuum sensors are included to sense when each hopper is discharged, and to transmit sensed information to the controller. When a discharged condition is sensed, the discharge controller is set to close the valve of the discharge hopper and open the valve of the next hopper in sequence. In some boilers, the valves are modulated instead of being fully opened and closed. As used herein, opening and closing of the valves is intended to encompass this alternative.
Ash hoppers do not fill with ash at the same rate. However, it is necessary to set the timing of the opening sequence to ensure that a hopper does not fill beyond its capacity between discharge times. Thus, since the hoppers are opened in sequence regardless of the amount of ash that is in the individual hoppers, many hoppers are unnecessarily opened when there is very little ash to discharge.
Opening and closing of hopper gate valves consumes energy, and causes wear on the gates. Further, sequencing of discharges from hoppers that are only partially filled reduces the efficiency of the overall system. Thus, an apparatus and method for opening boiler gate valves only when the hoppers are substantially filled would be of considerable value.
The present invention addresses this need by providing an apparatus and method for controlling the opening sequence of fossil-fired boiler ash hoppers, and fossil-fired boilers incorporating the apparatus and utilizing the method.
Generally, the apparatus of the present invention is designed for use with a fossil-fueled boiler having a plurality of adjacent ash collection hoppers, each hopper including a gate valve and a valve actuator to open and close the gate valve. The boiler will also include a conveyor for transferring ash from the hoppers to a remote destination for disposal or further processing. The conveyor, for example, may be a primary conduit communicating with the hoppers through secondary conduits that join the hoppers to the primary conduit through the gate valves. A vacuum source may be present to draw ash from the hoppers through the secondary and primary conduits. It will be apparent to one skilled in the art after reading the present description that the apparatus of the present invention can be used with fossil-fueled boilers of other constructions, and boilers with additional features, such as additional valves in the discharge conduits, e.g., branch valves at the juncture of the secondary and primary conduits.
The present apparatus is comprised of at least one sensor to measure the length of time required to discharge each of the hoppers; and a controller to receive information from the sensor or sensors and send control signals to the actuators dependent upon the sensed times. As used herein, the term xe2x80x9cdischargexe2x80x9d and derivations thereof, is intended to refer to the act of removing a desired amount, normally substantially all, of the ash from a hopper. The discharging of a hopper may collectively involves several actions, including the opening of the hopper""s gate valve and any other valves in the discharge conduit, and the application of a suction to the hopper to draw ash into the conduit.
The controller is initially programmed to open each hopper valve in sequence, with each hopper being opened for a predetermined time. When the valve of a given hopper is opened, a sensor measures the time required to discharge the hopper. This sensor may be unique to the given hopper, e.g., each hopper may have its own sensor. Alternatively, a common sensor may be used to measure the times required to open all hoppers. For example, a vacuum sensor, similar to that mentioned above, can be used to measure the time required to discharge a given hopper by determining the time when the valve is opened, and the time when the hopper resistance drops below a given value.
The controller is programmed with a desired discharge time range determined by a minimum discharge time and a maximum discharge time. The maximum discharge time will normally be equal to, or slightly less than, the time that the hopper valve is open during a cycle, while the minimum discharge time will be less than the maximum discharge time.
If the time required to discharge a given hopper is within the desired time range, no adjustment is made to the system. However, if the time required to discharge the hopper is less than the minimum discharge time, the opening sequence will need to be adjusted to avoid unnecessary opening of the given hopper. Therefore, the controller adds a time increment to the given hopper. The length of this time increment will depend on various factors, including the number of hoppers being discharged, the time required to discharge a full hopper, and the time that the sensor indicates as actually required to discharge the given hopper.
During the next and subsequent cycles, opening of the given hopper valve is skipped, until the assigned time increment expires. The given hopper valve is then discharged on the next cycle. If the hopper is discharged in a time within the range, no adjustment is made, and the existing time increment is reassigned. However, if the hopper is again discharged in a time less than the minimum discharge time, an additional time increment will be added to the initial time increment for the given hopper to create a second or revised time increment that is used to determine the time of the next discharge. Opening of the hopper will then be skipped for one or more cycles until the revised time increment, i.e., the sum of the initial time increment and the added time increment expires. The added time increment is not a fixed time or percentage and may vary depending upon various factors including those noted previously, e.g., the number of hoppers being discharged, the time required to discharge a full hopper, and the time that the sensor indicates as actually required to discharge the given hopper.
This procedure will be repeated on subsequent cycles until the time required to discharge the hopper falls within the desired range. Similar adjustments will be made for all hoppers. Thus, when the system reaches xe2x80x9csteady-statexe2x80x9d operation, each hopper that does not discharge within the desired discharge time range will have a time increment assigned by the controller, so that any given hopper will be bypassed during discharge cycles until the hopper""s assigned time increment has expired. The hopper is then discharged, and the existing or revised time is assigned.
In some systems, there may be a risk that a hopper will overflow if the actual time required to discharge the hopper exceeds the time that the hopper gate valve is opened on a given cycle. In order to avoid this risk, the timing of the cycles and the hopper opening time within a cycle is preferably set so that all hoppers are discharged in less than the preset time that the hopper is open. Then, a time increment will be added to each hopper, so that none of the hoppers will be opened for one or more subsequent cycles.
On subsequent cycles when the hoppers are again opened, some of the hoppers may discharge within the desired time range. The time increment for these hoppers will be maintained. Some hoppers may discharge in less than the minimum discharge time. The time increment for these hoppers will be increased. Some hoppers may not discharge by the maximum discharge time. The time increment for these hoppers will be reduced.
The assigned time increments will then be maintained, increased or decreased on subsequent cycles until all hoppers are discharged within the desired time range. The time to discharge each hopper will then be monitored on each subsequent cycle in which the hopper is opened. If operating conditions change so that the time required to discharge a given hopper increases or decreases, the given hopper""s assigned time increment will be adjusted accordingly.
Thus, the sequence for any given hopper comprises the initial step of discharging the hopper, comparing the actual time required to discharge the hopper with a desired time range having a minimum time and a maximum time, assigning a time increment to the given hopper if the actual time is less than the minimum time, and discharging the hopper again on a subsequent cycle only after the assigned time increment has expired. If the actual time required to discharge the hopper is above or below the desired time range when the hopper is discharged on the subsequent cycle, the assigned time increment is increased or decreased accordingly.
When there is a need to ensure that a given hopper does not overflow because the actual time to discharge the hopper significantly exceeds the actual hopper opening, the frequency of the cycles and the desired time range will be set to ensure that the actual time to discharge the given hopper on an initial cycle will be less than the minimum time of the desired time range. As a result, an increment of time will be assigned to the given hopper, so that the given hopper will be skipped or bypassed on one or more subsequent cycles. Then, if the actual time required to discharge the given hopper on a subsequent cycle is greater than the upper limit of the desired range, the time increment can be decreased to reduce the number of skipped cycles.
Thus, it is an aspect of the present invention to provide a method for discharging ash from a plurality of fossil-fuel boiler hoppers by opening only selected hoppers during a given discharge cycle and bypassing other hoppers comprising assigning initial time increments to given hoppers based on a comparison of the actual time required to discharge the given hopper and a desired time range, and bypassing opening of hoppers having unexpired assigned time increments.
It is another aspect of the invention to provide an apparatus for discharging ash from a plurality of fossil-fueled boiler hoppers, each having a gate valve comprising an actuator associated with each gate valve, at least one sensor to detect when each hopper is discharged, and a controller regulating opening of selected gate valves during a given discharge cycle, the gate valves being selected based on information received from the sensor.
These and other aspects of the present invention will become apparent to one skilled in the art upon reading the detailed description of the invention that follows, taken together with the drawings.